It is well established that water-insoluble, thixotropic, gel-like compositions can be used in blood collection tubes to form a barrier between centrifuged red blood cells and the supernatant serum, thereby permitting the ready removal or isolation of the serum without contamination by the red blood cells. See, e.g., U.S. Pat. Nos. 3,780,935; 3,852,194; 3,920,549; and 4,021,340, which teach the use of silicone fluids or polybutenes (or polyisobutenes) in combination with appropriate fillers as the thixotropic, gel-like compositions which are suitable for use in such collection tubes.
Because the specific gravities of the centrifuged red blood cells and the supernatant serum are in the ranges of from about 1.092 to about 1.095 and from about 1.026 to about 1.031, respectively, the gel-like compositions employed to separate the two phases will have specific gravities of from about 1.03 to about 1.09 and preferably from about 1.037 to about 1.05. Thus, the amount of filler employed with any given liquid component is, in essence, fixed.
It should be apparent, therefore, that the rheological properties of the prior art gel-like compositions generally can be varied or controlled only by changes in the composition components. That is, for any given gel-like composition, the rheological properties of the composition are, as a practical matter, predetermined by the specific gravities of the liquid component and the filler and the viscosity of the liquid component.
Because of the nature of the interactions involved in such thixotropic, gel-like compositions, the rheological properties can be varied to some extent by employing two or more fillers having different affinities. As used herein, the term "affinity" (or variations thereof) refers to the number of hydroxy groups present on the filler surface per unit area. Because composition viscosity results largely from interactions between the liquid component and filler surface hydroxy groups, composition viscosity can be altered by changing overall or total filler affinity. Similarly, such changes affect the thixotropic index of the composition since thixotropy results largely through particle-particle interactions via filler surface hydroxy groups.
As a practical matter, however, the use of more than one filler introduces a new problem, that of "wet-out". As used herein, "wet-out" refers to the loss of composition viscosity with time, with the concomitant separation of small amounts of the liquid component. In fact, wet-out can occur with compositions prepared from a silicone fluid and a single filler, such as silica, and U.S. Pat. No. 4,049,692 is directed to preventing wet-out in this latter situation. According to the patent, wet-out is prevented by incorporating in the composition an effective amount of a polysiloxanepolyoxyalkyl copolymer which effectively increases and stabilizes composition viscosity.
There still is a need, however, for a simple, effective, and reproducible means of controlling the rheological properties of the thixotropic, gel-like compositions employed in blood separation tubes, particularly when composition viscosities or thixotropic indices are too high.
It should be noted that the separate treatment of a filler, e.g. silica, with a disilazane, e.g. hexamethyldisilazane, does not constitute a part of the present invention. More specifically, the treatment of silica with hexamethyldisilazane is known. For example, U.S. Pat. No. 3,600,326 discloses the preparation of hydrophobic silica by milling a mixture of colloidal silica, Skellysolve F, and hexamethyldisilazane and then flashing off the solvent. Furthermore, the kinetics of the reaction of hexamethyldisilazane with the hydroxy groups on the surface of silica have been studied; see M. L. Hair et al., J. Phsy. Chem., 75, 2181 (1971) [Chem. Abstr., 75, 67966m (1971)].
It also should be noted that colloidal silica which was pretreated with either hexamethyldisilazane or a fluorosilane has been employed to prepare a silicone fluid-based lubricating grease. See U.S. Pat. No. 3,526,594. Such pretreated silica also is known to be useful in chromatography.
These prior art procedures all involve the pretreatment of silica. The references do not disclose or suggest the in situ use of a disilazane to modify a filler having surface hydroxy groups in order to control the rheological properties of a water-insoluble, thixotropic, gel-like composition which is suitable for use in blood collection tubes.